Preparation of aromatic carboxylic acids

ABSTRACT

IN A PROCESS FOR PREPARING A SPECIFIC AROMATIC CARBOXYLIC ACID IN A LOWER ALIPHATIC CARBOXYLIC ACID SOLVENT UNDER AN OXYGEN PARTIAL PRESUURE OF 0.2-70 ATM. AND AT A TEMPERATURE OF 60-200*C., AN IMPROVEMENT WHICH COMPRISES USING A COBALT COMPOUND AS A CATALYST AND PARALDEHYDE AS A PROMOTER, CHARACTERIZED IN THAT THE AMOUNT OF SAID COBALT COMPOUND IS ABOUT 0.62-4% BY WEIGHT AS COBALT BASED ON SAIID LOWER ALIPHATIC CARBOXYLIC ACID SOLVENT AND THE TOTAL USING AMOUNT OF SAID PARALDEHYDE IS ABOUT 0.667-8% BY WEIGHT OF SAID LOWER ALIPHATIC CARBOXYLIX ACID SOLVENT.

3,636,095 Patented Jan. 18, 1972 3,636,095 PREPARATION OF AROMATIC CARBOXYLIC ACIDS Kenji Nakaoka, Numazu-shi, and Tadao Kato and Seikichi Matsuhisa, Mishima-shi, Japan, assignors to Toyo Rayon Kahushiki Kaisha, Tokyo, Japan No Drawing. Continuation-impart of application Ser. No. 330,220, Dec. 2, 1963. This application July 10, 1967, Ser. No. 652,012

Int. Cl. C07c 63/02 US. Cl. 260524 R 12 Claims ABSTRACT OF THE DISCLOSURE In a process for preparing a specific aromatic carboxylic acid in a lower aliphatic carboxylic acid solvent under an oxygen partial pressure of O.2-7O atm. and at a temperature of 60-200 C., an improvement which comprises using a cobalt compound as a catalyst and paraldehyde as a promoter, characterized in that the amount of said cobalt compound is about 0.624% by weight as cobalt based on said lower aliphatic carboxylic acid solvent and the total using amount of said paraldehyde is about 0.667-8% by weight of said lower aliphatic carboxylic acid solvent.

This application is a continuation-inpart application of Serial No. 330,220 filed on Dec. 2, 1963 now abanoned.

The present invention relates to a process for preparing aromatic carboxylic acids. More particularly, the present invention relates to a process for preparing aromatic carboxylic acids which comprises oxidizing an aromatic compound having at least one side-chain alkyl group in the liquid phase by means of a molecular oxygen-contain ing gas in the presence of a lower aliphatic carboxylic acid as a solvent, a cobalt compound as a catalyst and paraldehyde as a reaction promoter.

The process for preparing aromatic carboxylic acids by oxidizing aromatic alkyls in liquid phase using molecular oxygen in the presence or absence of a solvent has been noted heretofore, with the consequence that numerous processes therefor have been proposed. However, while in most of these prior art processes it was relatively easy to obtain aromatic monocarboxylic acids by oxidizing the aromatic dialkyls, it was a diflicult matter to oxidize this monocarboxylic product further to obtain the aromatic dicarboxylic acids. Namely, while it has been proposed, when carrying out the liquid phase oxidation of aromatic dialkyls with oxygen, to use as catalysts various metals having variable valences, such as cobalt, manganese, chromium, nickel, lead and vanadium, the aromatic dicarboxylic acids can hardly be obtained with the use of these metal catalysts alone, because oxidation ceases at the stage in which the aromatic monocarboxylic acids are formed.

In order to oxidize the aromatic dialkyls to the aromatic dicarboxylic acids with oxygen in one step, it is necessary to use certain reaction promoters besides the aforementioned metal compounds. As these reaction promoters, bromine compounds (US. Pat. 2,833,816), ketones having methylene radicals adjacent carbonyl radicals (US. Pat. 2,853,514) aliphatic aldehydes such as acetaldehyde (U.S. Pats. 2,673,217 and 3,240,803) or organic compounds of a certain kind (US. Pat. 2,245,528) are known. However, although the process using bromine compounds is an excellent liquid phase air oxidation process with respect to its reaction rate and yield, its major defect lies in corrosion of the reaction equipment and qualitative deterioration of the product due to the use of bromine compounds. Other processes are free from corrosion of the reaction equipment and contamination of the product, however, they have defects that the amount of the promoter used is large and the yield is low. For instance, in the process of using ketones, pure oxygen is required as a source and inexpensive oxidizing agents such as air cannot be used, in the process using an aldehyde it is necessary to use a large amount of the aldehyde based on a substance to be oxidized, and in the process using organic compounds of a certain kind, the yield is very low.

It is a primary object of the present invention to provide an improved process in which the defects relative to the reaction rate, yield and the quality of the product such as possessed by the aforementioned prior art processes are eliminated when synthesizing aromatic carboxylic acids, particularly aromatic dicarboxylic acids, from the aromatic alkyls by means of the liquid phase oxidation process.

Another object of the persent invention is to provide a novel catalyst system for use in such an improved process.

It has been found that said objects of the present invention can be achieved by using a cobalt compound as a catalyst, paraldehyde as a reaction promoter and a lower aliphatic carboxylic acid as a solvent in oxidizing aromatic alkyls with molecular oxygen or 1 gas containing molecular oxygen.

In a preferred embodiment of the present invention a solution dissolving monoalkyl benzene, dialkyl benzene. monoalkyl benzaldehyde or monoalkyl benzoic acid in a fatty acid having 2 to 4 carbon atoms is contacted with a gas containing molecular oxygen in the presence of a cobalt compound and paraldehyde.

Paraldehyde used as a promoter in the present invention is known to be a cyclic trimer of acetaldehyde and when heated under certain conditions, for instance, in the presence of a small amount of sulfuric acid, it is easily decomposed to acetaldehyde [Encyclopedia of Chemical Technology, vol. 1, p. 42 (1947)]. From such fact, there is a fear of misunderstanding that paraldehyde used in the present invention is first dissociated to acetaldehyde under the existing reaction conditions, and that this acetaldehyde displays a promoting action. Therefore, in order to eliminate such misunderstanding differences between paraldehyde and acetaldehyde will be clarified hereinbelow.

Some physical and chemical properties of paraldehyde and acetaldehyde will be shown in the following table.

Acetaldehyde Paraldehyde Melting point, C 123.5 12.5. Boiling point, C 20.8

[According to Encyclopedia of Chemical Technology, vol. 1, Pp- 3234, 42 (1947)].

When the two are first compared with respect to their physical properties, the boiling point of paraldehyde is 124 C., in contrast to the boiling point of acetaldehyde of 21 C. Because the reaction temperature in the present invention is 60 C. or higher it is much higher than the boiling point of acetaldehyde, when acetaldehyde is used as a promoter, the greater part thereof evaporates, the amount thereof existing in the reaction liquid becomes very small, failing to show a satisfactory promoting action. Because paraldehyde has a relatively high boiling point, such disadvantage is eliminated and paraldehyde can develop a desired catalytic activity.

On the other hand, when the chemical properties of the two are observed, in contrast to acetaldehyde which shows a reactivity inherent in aldehydes, because paraldehyde is a cyclic ether, its chemical properties are essentially different from those of acetaldehyde; for instance, it fails to show the reactivity inherent in aldehyde. This point is considered a very important factor in explaining the characteristics of the present invention; however, a detailed explanation will be made later. From the foregoing facts, it will be understood that because paraldehyde is easily decomposed to acetaldehyde under conditions of a certain kind, seemingly it shOWs reactivity same as that of acetaldehyde sometimes, however, paraldehyde is essentially heterogeneous in physical and chemical properties.

Next, when comparison is made between a case wherein an oxidation reaction of aromatic alkyls is carried out by using paraldehyde as a reaction promoter as in the present invention and a case wherein said reaction is carried out by using acetaldehyde, the difference between the two becomes clearer (concerning the data, reference may be made to the examples to be mentioned later). Upon comparison, p-toluic acid is used as a substance to be oxidized, this is because, as mentioned above oxidation of the first alkyl radical of an aromatic alkyl proceeds satisfactorily in the presence of a heavy metal catalyst such as a cobalt compound only, however, for oxidation of a second alkyl radical a promoter of a certain kind is necessary, and in order to clarify the difierence in effect of said promoter, it is considered optimum to compare by using p-toluic acid which is an already oxidized aromatic alkyl. When the promoting etfects of paraledhyde and acetaldehyde are compared as such, the diiference between their effects is very clear. Namely, in case paraldehyde is used, terephthalic acid is produced in a relatively high conversion, in contrast, when the same amount of acetaldehyde is used instead of paraldehyde, either p-toluic acid is recovered almost unreacted or even when it is reacted, its conversion is very low. In fact, in a known process wherein acetaldehyde is made a promoter (US. Pat. 2,673,217) acetaldehyde is used in an amount exceeding the amount of a substance to be oxidized and unless such a large amount of acetaldehyde is used it is not possible to carry out the reaction smoothly. In contrast thereto, paraldehyde shows an excellent promoting action even in a small amount of below 1 mole based on 1 mole of the substance to be oxidized.

As mentioned above, in an oxidation reaction such as the present invention, paraldehyde shows an action entirely diiferent from that of acetaldehyde and which action is excellent. This fact cannot be explained unless it is considered that a mechanism whereby paraldehyde is first decomposed to acetaldehyde in the reaction system and this acetaldehyde shows a promoting action is not taken, but the main process is decomposition without processing through acetaldehyde. It is considered that paraldehyde or acetaldehyde in this reaction show the following functional mechanism. Namely:

Paraldehyrle or Peroxide Acetaldchyde Peroxide Co Co Pat-aldehyde and acetaldehyde are first oxidized to a peroxide (perhaps peracetic acid), which peroxide oxidizes C++ to Co+++ according to reaction (2), and the resulting Co+++ is considered to carry out a hydrogen ex- 4. traction reaction according to reaction (3). In this case, it is considered that parallel to these reactions, a reaction whereby active Co+++ changes to inactive Co like reaction (5) occurs:

However, the occurrence of such reaction is undesirable. That the promoting eifect of acetaldehyde is small as compared with that of paraldehyde is inferred due to the aforementioned difference of boiling points, and the fact that the reaction of the Formula 5 is apt to occur by reason of the reducing properties of the aldehyde group.

As aromatic compounds having at least one side-chain alkyl group used as a material substance in the present invention, for instance, toluene, xylenes and cymenes are preferable, materials having a substituent other than the alkyl group such as the carboxyl group and the aldehyde group besides the alkyl group, such as toluic acids and tolualdehydes, are also useful.

As a lower aliphatic carboxylic acid used as a solvent, aliphatic carboxylic acids having 2 to 4 carbon atoms are preferable, above all acetic acid and propionic acid, especially acetic acid, are preferable. The aforementioned aromatic compound is subjected to oxidation as a solution dissolved in said aliphatic carboxylic acid, preferably at a concentration of about 5-60% by weight based on a solvent.

The cobalt compound used as a catalyst is preferably one which dissolves in the aforementioned lower aliphatic carboxylic acid used as a solvent under the reaction con ditions, for instance, a cobalt salt of said lower aliphatic carboxylic acid or a cobalt salt of an acid weaker than such carboxylic acid is used. As a concrete example, cobalt acetate or cobalt carbonate may be cited. The amounts of these catalysts to be added are preferably amounts corresponding to (LOGS-5% by weight of cobalt based on said lower aliphatic carboxylic acid solvent.

Pat-aldehyde as a promoter is preferably added in an amount of 02-30% by weight, based on said lower aliphatic carboxylic acid solvent.

Molecular oxygen or a gas containing molecular oxygen used as an oxidizing agent is conveniently used under an oxygen partial pressure of 0.27() atmospheres at a temperature of 20() C.

In order to assist the dissolution of the catalyst, at small amount of water may be added to the reaction system.

As mentioned above, characteristics of the present invention reside in the use of combination of a cobalt compound with paraldehyde, by this combination only the desired excellent effect is developed, and in case paraldehyde is combined with a metal compound other than a cobalt compound, for instance, a manganese compound and in case paraldehyde is replaced by another compound, the excellent catalytic elfect is not observed.

The present invention is not limited in reaction form and can be practiced either batchwise or by a continuous process, however, a continuous process is more advantageous.

When the process of the present invention is practiced batchwise, if all the paraldehyde is added at one time at the outset of the reaction, because pal-aldehyde is more apt to be oxidized than aromatic alkyls, oxidation of aromatic alkyls to aromatic carboxylic acids does not com pletely take place. Therefore, if it is necessary to completely convert the aromatic alkyls to aromatic carboxylic acids, it is recommended to supply a predetermined amount of paraldehyde continuously or intermittently throughout the entire reaction period. In this case, it is preferable to supply paraldehyde in an amount corresponding to 0.005-5% by weight based on a lower aliphatic carboxylic acid solvent per 10 minutes continuously or intermittently.

If as a starting material, a substance which is difiicult to oxidize such as toluic acids, is used in carrying out the reaction under mild conditions, it is possible to shorten the induction period when aromatic aldehydes are added as a reaction initiator at the outset of the reaction. As examples of these aromatic aldehydes, it is possible to illustrate by mentioning benzaldehyde, tolualdehydes, carboxybenzaldehydes and phthaloaldehydes, the amount of which is preferably 0.005% by weight based on the aliphatic carboxylic acid solvent.

When the process of the present invention is practiced as a continuous process, the material aromatic alkyls, the cobalt compound catalyst, the paraldehyde promoter and the lower aliphatic carboxylic acid solvent are continuously supplied at said ratio to an oxidation reactor to contact with molecular oxygen or a gas containing molecular oxygen. The aromatic carboxylic acid produced is separated by the conventional method. From the residual liquid, water and acetic acid (produced by oxidation of paraldehyde) are produced as byproduct during the re action and are removed. The resulting lower aliphatic carboxylic acid solution containing cobalt per se is circulated for re-use. Or it is possible to separate and recover cobalt and a lower aliphatic carboxylic acid from said solution by a proper method and use them respectively in the reaction again. For instance, in case as material p-xylene is used and as solvent acetic acid is used, the present invention can be conveniently carried out by removing water and excess acetic acid from a mother liquor after filtering terephthalic acid produced by the oxidation reaction, supplying the obtained acetic acid solution containing cobalt again to the reactor, and newly supplying p-xylene, paraldehyde and an additional amount of a cobalt compound as occasion demands simultaneously.

The present invention is further illustrated by the following non-limitative examples.

EXAMPLE 1 A mixture of 13.6 g. of p-toluic acid, 2.44 g. of cobalt acetate, 2 g. of paraldehyde and 100 g. of acetic acid was heated to 95l00 C., following which air was passed therethrough at the rate of 300 ml./min. The reaction liquid which at first was pink turned dark green 5 hours later, following which a white precipitate of terephthalic acid gradually separated to obtain 3.0 g. of terephthalic acid after 18 hours.

When paraldehyde was not added, the reaction liquid remained dark red in color and no terephthalic acid whatever was obtained even after 18 hours.

EXAMPLE 2 Instead of the ptoluic acid in Example 1, 10.6 g. of p-xylene was used and after adding 2 g. of water to the reaction liquid for assisting the solution of the cobalt acetate, the oxidation was carried out as in Example 1. After 3.5 hours, the reaction liquid turned dark green and after 6 hours, terephthalic acid started to separate as a white precipitate to yield 18 hours later 4.1 g. of p-toluic acid and 2.6 g. of terephthalic acid.

EXAMPLES 3-6 A 100 cc. shaking type stainless steel autoclave was charged with 2 g. of p-toluic acid, 0.4 g. of cobalt acetate, 0.8 g. of water, 15 g. of acetic acid, and paraldehyde in the amounts indicated in Table I. Into the autoclave was then introduced under pressure oxygen gas to 20 atmospheres, after which the autoclave was raised to a temperature of 130 C. in about 40 minutes with shaking, at which temperature it was held for 2 hours. After completion of the reaction, the reaction product was washed in a sodium hydroxide aqueous solution and after removing the precipitated cobalt hydroxide, it was separated by addition of hydrochloric acid washed with water and dried. Then by measuring the acid value and yield the rate of conversion was obtained. Example 3 is a control which was carried out without the addition of paraldehyde.

TABLE I Amount Conver- Paralseparated sion to Yield of dehyde, by adding Acid TPA", TPA, Example g. acid, g. value percent percent TPA: terephthalic acid.

EXAMPLE 7 This example is a control of Example 5. When Example 5 was repeated using 0.4 g. of manganese acetate instead of the cobalt acetate only 1.81 g. of a product separated by addition of acid having an acid value of 415.1 was obtained. This indicates that practically no oxidation to terephthalic acid occurred.

EXAMPLE 8 When the oxidation was carried out following the same procedure as described in Example 6, except that 1.8 g. of p-xylene was used as the starting material instead of the p-toluic acid and the initial pressure used for the oxygen was 40 atmospheres, 1.46 g. of p-toluic acid and 0.42 g. of terephthalic acid were obtained. In a control in which paraldehyde was not added, only 0.18 g. of p-toluic acid was obtained.

EXAMPLE 9 When Example 6 was repeated except that instead of the p-toluic acid 2 g. of m-toluic acid was used as the starting material, 0.89 g. of isophthalic acid was obtained.

EXAMPLE 10 When the same procedures as described in Example 6 were followed in carrying out the oxidation, except that as the starting material 1.4 g. of toluene was used instead of the p-toluic acid 0.87 g. of benzoic acid was obtained.

In a control in which paraldehyde was not added, only 0.09 g. of benzoic acid was obtained.

EXAMPLES 1114 When the reactions were carried out under the same condition as described in Examples 3-6 using mixtures consisting of 2 g. of p-toluic acid, 0.4 g. of the various classes of cobalt compounds shown in Table II, 0.3 g. of paraldehyde, 0.3 g. of water and 15 g. of acetic acid, the results are as shown in Table II.

A cc. shaking type stainless steel autoclave was charged with 2 g. of p-toluic acid, 0.4 g. of cobalt acetate, 0.1 g. of paraldehyde, and 15 g. of acetic acid. Into the autoclave was then introduced under pressure oxygen gas to 20 atmospheres, after which the autoclave was raised to a temperature of 100 C. in about 30 minutes while shaking it and thereafter, while adding at each interval of 45 minutes 0.1 g. of paraldehyde, was held at 100 C. for 4.5 hours (270 minutes).

After completion of the reaction, the reaction product was washed in a sodium hydroxide aqueous solution and, after removing the precipitated cobalt hydroxide, was separated by adding hydrochloric acid, washed with water and dried, following which the rate of conversion to terephthalic acid was obtained by measuring the acid value and yield. The results are shown in Table III.

This is a control for showing the effectiveness of the incremental addition of the paraldehyde in Example 15. When the reaction was carried out as in Example 15 for 4.5 hours but by charging 0.6 g. of the paraldehyde at the beginning and without any addition thereof subsequently, 2.19 g. of a product separated by adding acid having an acid value of 591.6 was obtained. Therefore, the rate of conversion to terephthalic acid in this case was 61% and the yield was 94.0%.

EXAMPLE 17 A 100 cc. shaking type stainless steel autoclave was charged with 15.6 g. of p-xylene, 0.4 g. of cobalt acetate, 0.3 g. of water and 15 g. of acetic acid. Into the autoclave was then introduced under pressure oxygen gas to 20 atmospheres, after which it was raised to a temperature of 100 C. in about 30 minutes while shaking it and thereafter was held at this temperature for 2 hours. This was then followed by holding the autoclave at a temperature of 100 C. for 4 hours while adding at each 30 minute interval 0.1 g. of paraldehyde. The reaction product, after being treated as described in Example 15, showed This is a control to show the eifectiveness of the incremental additions of paraldehyde in Example 17. When the reaction was carried out as in Example 17 for 4 hours but by adding 0.4 g. of the paraldchyde at the beginning and without any additions thereof subsequently, 1.82 g. of a product separated by adding acid having an acid value of 495.2 was obtained. Therefore, the yield of the terephthalic acid was 23.5% in this case while that of p-toluic acid was 62.5%.

EXAMPLE 19 13.6 g. of p-toluic acid, 0.62 g. of cobalt acetate, 0.5 g. of benzaldehyde and 100 g. of acetic acid were heated to 95100 C., and then the reaction was begun by passing air through at the rate of 300 ml./ min. at normal atmospheric pressure. The reaction liquid which was dark red at first turned dark green 1.5 hours later. When the reaction was continued for 20 hours while adding after the two hours 0.1 g. of paraldehyde at each one hour interval (the total paraldehyde addition being 1.8 g), 5.9 g. of terephthalic acid was obtained, the reaction liquid maintaining its dark green color to the end.

EXAMPLE 20* When the reaction in Example 19 was continued with- F out adding the paraldehyde, the color of the reaction liquid reverted to dark red from its dark green color after 7 hours, and even though the reaction was carried out for 13 hours, only 0.6 g. of terephthalic acid was obtained.

8 EXAMPLE 21 Although 13.6 g. of p-toluic acid, 2.49 g. of cobalt acetate, 0.5 g. of paraldehyde and g. of acetic acid were heated to 95100 C. and air was passed through for 15 hours at the rate of 300 mL/min. at normal atmospheric pressure, the reaction liquid remained dark red in color and no terephthalic acid .at all was obtained.

EXAMPLE 22.

When Example 21 was repeated except that the amount used of the paraldehyde was made 2 g., the reaction liquid turned dark green 5 hours later and after 18 hours, 3.0 g. of terephthalic acid was obtained.

EXAMPLE 23 Example 19 was repeated except that after 2 hours had elapsed after the reaction began, the reaction was continued for 20 hours while adding instead of the paraldehyde at each one hour interval 0.1 g. of the additives indicated in Table V (total amount of additions being 1.8 g.). The amounts formed of terephthalic acid are shown in Table V.

TABLE V Additive: Terephthalic acid, g. Benzaldehyde 3.1 Chloroacetone 2.5 Methylethyl ketone 1.8 Methylisobutyl ketone 0.7

EXAMPLE 24 Except that the cobalt acetate, water and acetic acid were reduced respectively to 0.2 g., 0.15 g. and 7.5 g. and the interval in which the paraldehyde was added was extended to 45 minutes, the reaction was carried out exactly as in Example 17. The results are as shown in Table VI.

TABLE VI Total amount of Amount p-Toluic paralseparated acid TPA Reaction dehyde by adding Acid yield, yield, time, hr. added, g. acid, g. value percent percent EXAMPLE 26 Example 25 was repeated except that the cobait acetate, water and acetic acid were reduced respectively to 0.1 g, 0.1 g, and 5 g. The results are shown in Table VII.

TABLE VII Total amount of Amount p-Toluie paralseparated acid TPA Reaction dehyde by adding Acid yield, yield, time, hr. added, g. acid, g. value percent percent EXAMPLE 27 20 g. of p-toluic acid, 4 g. of cobalt acetate, 2 g. of paraldehyde and g. of acetic acid were placed in a stainless steel gas circulating type pressure reaction tube 1 inch inside diameter and into this tube oxygen was introduced under pressure to 20 atmospheres. While adjusting the rate of discharge of oxygen to become 100 ml./min. under standard conditions, the blowing in of the oxygen was continued, Maintaining a liquid temperature of 95-105 C., 2 g. of paraldehyde was added when 1.5 hours had elapsed after the start of the reaction and likewise when 3 hours had elapsed, after which the reaction was continued for another 1.5 hours. Thus, the amount of paraldehyde added totaled 6 g. while the total reaction time was 4.5 hours. After completion of the reaction, a large quantity of a sodium hydroxide aqueous solution was added to the reaction product and after removing the precipitated cobalt hydroxide, the product was separated by adding hydrochloric acid whereupon was obtained 22.74 g. of a product separated by adding acid having an acid value of 655.5. Thus, the rate of conversion to terephthalic acid was 86.1% and the yield was 94.5%.

EXAMPLE 28 Using 31.8 g. of p-xylene, 4 g. of cobalt acetate, 3 g. of water, 2 g. of paraldehyde and 100 g. of acetic acid, the reaction was carried for 2 hours at 95105 C. following the same procedures as described in Example 27, except that further addition of paraldehyde was not made, As a result, 26.5 g. of a product separated by adding acid having an acid value of 441.8 was obtained. Thus, the yield of terephthalic acid was 6.1% and that of p-toluic acid was 60.1%.

EXAMPLE 29 A 500 cc. shaking type stainless steel autoclave was charged with 15.6 g. of p-xylene, 2 g. of cobalt acetate, 1.5 g, of water, 2 g. of paraldehyde and 75 g. of acetic acid, and into it was then introduced under pressure oxygen gas to atmospheres. After raising the temperature to 95-105 C. in about 30 minutes and then maintaining this temperature for 2 hours, further addition of 2 g. of paraldehyde was made, followed by the introduction under pressure anew of oxygen gas to 20 atmospheres, the autoclave being held at this temperature for another hour. Thus, the total amount added of the paraldehyde was 4 g. and the total reaction time was 3 hours. When the reaction product was treated as in Example 27, 22.1 g. of a product separated by adding acid having an acid value of 623.1 was obtained. Therefore, the yield of terephthalic acid was 72.6% and that of p-toluic acid was 22.0%.

EXAMPLE 30 A 100 cc. shaking type stainless steel autoclave was charged with 2 g. of m-xylene, 7.5 g. of n-butyric acid, 0.2 g. of cobalt acetate and 0.4 g. of paraldehyde, into said autoclave oxygen gas was pressed until the pressure inside the autoclave became 20 atmospheres, while shaking the autoclave the temperature inside was raised to 110 C. in about 30 minutes, and said temperature was maintained for 3 hours. After the reaction the product was washed with sodium hydroxide aqueous solution, the precipitated catalyst was removed and the product was separated by hydrochloric acid to obtain 1.52 g. of the product separated Whose isophthalic acid content was 52%.

EXAMPLE 31 In Example 30, 2 g. of p-tolualdehyde was used instead of said m-xylene and the similar reaction was carried out to obtain 2.05 g. of the product separated by hydrochloric acid whose terephthalic acid content was 62.3%.

EXAMPLE 32 In Example 30, 2 g. of m-tolualdehyde was used instead of said m-xylene and the similar reaction was carried out to obtain 1.91 g. of the product separated by hydrochloric acid whose isophthalic acid content was 49.2%.

10 EXAMPLES 33-35 In Examples 36, instead of paraldehyde acetaldehyde was used. The results were shown in Table VIII, from which table it is understood that conversion to terephthalic acid hardly took place.

EXAMPLES 3637 A glass atmospheric pressure oxidation reactor having an internal diameter of 2.5 cm. and a length of 50 cm. was charged with 6.8 g. of p-toluic acid, 0.63 g. of cobalt acetate, g. of acetic acid and 0.5 g. of benzaldehyde, while passing air thereinto at a rate of 200 cc. per minute, the temperature was raised to 100 C. in 10 minutes and the reaction was continued for 20 hours. The promoter shown in Table IX was added at a ratio of 0.1 g. per 1 hour from 2 hours after initiation of the reaction (total amount being 1.8 g.). The results were shown in Table IX.

In case acetaldehyde was used, when it was added dividedly, production of terephthalic acid was observed; however, that its amount was less than one half as compared with the amount of terephthalic acid produced when paraldehyde was used is understood from the above table.

EXAMPLE 38 A mixture consisting of 15 parts by Weight of p-xylene, 5 parts by weight of paraldehyde, 0.5 part by weight of cobalt acetate tetrahydrate and 79.5 parts of acetic acid was continuously supplied into a reactor maintained at C. and under 15 kg./cm. gauge pressure so that the average residence time inside the reactor might become 2 hours to contact said mixture with a mixed oxygen-nitrogen gas having an oxygen partial pressure of 8 kg./cm. blown in from the lower part of the reactor. The produced slurry reaction product containing terephthalic acid particles was continuously taken-out from the reactor, solid particles were filtered, washed with water and dried. Acid value of this product was 673.9 mg. KOH/g. and purity as terephthalic acid was more than 99%.

The amounts of cobalt compound by weight used in each of the examples, based on the weight of the solvent, are within the range of 0.624%, specifically:

2.44% (Examples 1 and 2),

2.67% (Examples 4-6, 813, 15-18, 24, 25, 27, 29, 30,

0.62% (Example 19),

2.49% (Examples 21-22),

2.00% (Example 26),

4.00% (Example 28), and

0.63% (Examples 36, 38).

The amounts of paraldehyde used by weight in each of the examples, based on the 'weight of the solvent, are within the range of .667% to 8%, specifically:

2% (Examples 1, 2),

.667% (Examples 4, 16), 3.33% (Example 5),

6.67% (Examples 6, 8, 9, l0),

667%, 1.33%, 2.00%, 2.67%, 3.33%, 4.0% (Example 1.33%, 2.67% (Example 17), 1.8% (Example 19), 2% (Example 22), 2.67% (Example 24), 2.67%, 5.34% (Example 25), 4.0%, 8.0% (Example 26), 4.0% (Example 27), 2% (Example 28), 5.34% (Examples 29, 30, 31, 32), 1.8% (Example 36) and 6.3% (Example 38).

The weight ratio of the aromatic compound having at least one side-chain alkyl group in the lower aliphatic carboxylic acid solvent is, as disclosed in the examples, in the range of 6.8% to 312% by weight, based on the weight of the solvent. Specifically, referring to the examples, the disclosed weight ratios are:

13.6% (Examples 1, 19, 21, 22), 10.6% (Example 2),

2/15=l3.4% (Examples 4-6, 9, 11-l3, l5, 16, 24), 1.8/15=12.0% (Example 8), 1.4/15=9.3% (Example 10), 15.6/l5=104% (Examples 17, 18), 15.6/7.5==208% (Example 25), 15.6/5=312% (Example 26), 20/150=l3.33% (Example 27), 31.8/100=31.8% (Example 28), 15.6/75=20.8% (Example 29), 2/7.5=26.67% (Examples 30, 31, 32), 6.8/l00=6.8% (Example 36), and 15/79.5=18.9% (Example 38).

What is claimed is:

1. In a process for preparing an aromatic carboxylic acid selected from the group consisting of benzene monocarboxylic acids and benzene dicarboxylic acids by oxidizing an aromatic compound having at least one side-chain alkyl group selected from the group consisting of monoalkyi benzenes, dialkyl benzenes, monoalkyl benzaldehydes and monoalkyl benzoic acids with a molecular oxy gen-containing gas in a lower aliphatic carboxylic acid solvent under an oxygen partial pressure of 0.2-70 atm. and at a temperature of 60-200 C., the improvement which comprises incorporating into the reaction mixture a catalytic quantity of a cobalt compound and further incorporating into the mixture paraldehyde as a promoter, characterized in that the total amount of said paraldehyde used is 02-30% by weight based upon the weight of said lower aliphatic carboxylic acid solvent.

2. The process defined in claim 1, wherein the amount of said lower aliphatic carboxylic acid solvent is an amount sufficient to dissolve said aromatic compound having at least one side-chain alkyl group at a ratio of 6.8% to 312% by weight, based on the weight of the solvent.

3. The process defined in claim 1, wherein said lower aliphatic carboxylic acid solvent has 2-5 carbon atoms.

4. The process defined in claim 1, wherein said aromatic compound having at least one side-chain alkyl group is a xylene.

5. The process defined in claim 1, wherein said cobalt compound is a cobalt salt of an acid which is weaker than said lower aliphatic carboxylic acid.

6. The process defined in claim 1, wherein the reaction medium is initially substantially anhydrous.

7. The process defined in claim 1, wherein the reaction medium initially contains water.

8. The process defined in claim 1, wherein said aromatic compound is p-toluic acid, the catalyst is cobalt acetate, and the solvent is acetic acid.

9. The process defined in claim 1, wherein the amount of par-aldehyde added is at least about 0.5% by weight.

10. The process defined in claim 1, wherein the amount of paraldehyde added is in the range of about 0.667% to about 8% by weight.

11. The process defined in claim 1, wherein the amount of cobalt compound is about 0.624% by weight of cobalt based on said solvent.

12. In a process for preparing an aromatic carboxylic acid selected from the group consisting of benzene monocarboxylic acids and benzene dicarboxylic acids by oxidizing an aromatic compound having at least one sidechain alkyl group selected from the group consisting of monoalkyl benzenes, dialkyl benzenes, monoalkyl benzaldehydes and monoalkyl benzoic acids in the presence of a molecular oxygen-containing gas in a lower aliphatic carboxylic acid solvent under an oxygen partial pressure of 02-70 atm. and at a temperature of 60200 C., and also containing a catalytic quantity of a cobalt compound, the improvement which comprises adding paraldehyde to the reaction mixture in separate quantities at difierent times during the reaction period, and controlling said paraldehyde addition so that the amount of paraldehyde added per 10-minute time period of the reaction is in the range of 0.0055% by weight based on the weight of said lower aliphatic carboxylic acid solvent.

References Cited UNITED STATES PATENTS 2,673,218 3/1954 Caldwell 260-524 3,361,803 1/1968 Augustynowicz 260--524 3,240,803 3/1966 Thompson et al 260-524 OTHER REFERENCES Fieser et al., Organic Chemistry, 1955, pp. 199-200.

LORRAINE A. WEINBERGER, Primary Examiner R. S. WEISSBERG, Assistant Examiner U.S. Cl. X.R. 260523 R 

